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We can probably limit our search somewhat, since there are some "obviously better" spots to farm. Here's my hit list of best Spartan Ops (only the first 3 episodes have been released, so this might need updating later...) For the purposes of this, I've played the op on Normal, solo, and killed any allied units that might steal kills. There may be some variance in the enemy types depending on the difficulty, but the time invested in playing Heroic/Legendary vs the XP/kills is probably not worth it.
Prometheans
For Prometheans, the "overall best" is currently E2C5 (Gagarin). On this op, there are:
40 Crawlers
3 Crawler Snipers
55 Crawler Primes
16 Knights
3 Knight Battlewagons
1 Knight Commander
8 Watchers
This one is made easier by the fact that there are incineration cannons on a rack on the second story of the central area. There's also a lot of shotguns and shotgun ammo, which on Normal will kill a Knight pretty quickly.
I believe there are more Watchers on E2C1 (Cleanup), but it takes a while to get to them so I wouldn't suggest farming Watchers here unless you've also got some Covenant enemies to farm.
Covenant
For Covenant, there are a couple of good ones. The one that I prefer to run is E3C4 (Shootout In Valhalla, aka "the Mantis one"), which has:
27 Grunts
17 Grunt Imperials
0 Grunt Ultras
24 Jackals
4 Jackal Majors
11 Jackal Rangers
32 Elites
0 Elite Zealots
19 Elite Officers
2 Elite Generals
12 Hunters
0 Ghosts
26 Banshees
1 Wraith
Halfway through, you'll gain a Mantis, and it will respawn if it is destroyed, as many times as you need it. Killing a Banshee also gives you credit for an elite, and they're pretty easy to down in a Mantis.
The Hunter count and the Banshee count are pretty insane. You'll note there's no Grunt Ultras, Elite Zealots or Ghosts on this map though. (Well, unless the Elites steal your Ghost...)
If you want some Grunt Ultras in your Spartan Ops, you might try E3C1 (The VIP, aka "the Scorpion one"):
28 Grunts
24 Grunt Imperials
7 Grunt Ultras
1 Jackal
5 Jackal Majors
25 Jackal Rangers
20 Elites
1 Elite Zealot
12 Elite Officers
7 Elite Generals
0 Hunters
0 Ghosts
0 Banshees
4 Wraith
About halfway through this level, you'll get another vehicle (TANK!) that respawns if it is destroyed. It's pretty easy to lay waste to everything with the tank. Even the early part is pretty easy, as after the first wave you'll get a drop of power weapons that make taking the hill fairly trivial.
We're still kind of low on Elite Zealots and Jackal Majors, so consider E3C5:
31 Grunts
19 Grunt Imperials
7 Grunt Ultras
13 Jackals
17 Jackal Majors
29 Jackal Rangers
3 Elites
14 Elite Zealots
1 Elite Officer
2 Elite Generals
2 Hunters
0 Ghosts
0 Banshees
0 Wraiths
NOTE: Playing this on normal significantly increases grunt ultras. There will be at least 11 before the second phase of the mission. So bear in mind if you are finished with the others and want to focus on ultras.
This one goes a lot slower, as there's not an abundance of vehicles or power weapons. I'd almost consider waiting it out for another Op that is somewhat easier and contains the enemies you need.
Other Notes
There's not a lot of Ghosts in my suggested hit list. There are some on E1C1 (Land Grab, includes multiple empty wraiths, ghosts, and one of each warthog), although there's a lot of dead space in that one and farming them solo's not great. Beyond that, killing Ghosts in War Games also counts towards this commendation, so you could also get progress by playing something like Big Team Slayer.
If I've got to farm just Hunters or Ghosts (ie, for a challenge), I'd probably go for E2C3 (Hacksaw). You can grab a rocket launcher at the beginning by sprinting straight ahead, and then there is more ammo at every location where the marines are under fire. Around the ramp up to the central structure are always 2 Hunters and 1 Ghost.
As a side note, if you're farming a particular type of enemy, you can kill that enemy and then restart the op and still get commendation/challenge credit for the kill.
Conclusion
While I understand you may want to farm commendations for the OCD value, the XP bonuses are very limited compared to the pain involved. I will tend to do the weekly challenges, but I don't give much time to worrying about the commendations, personally.
If you're just in it for XP, solo'ing will give you a better shot at getting a kill yourself, but you'll gain far more XP (2x or more!) by playing in a matchmade game.
